Currently engine components, in most instances, are fabricated with metallic materials with some substitution of ceramics or ceramic matrix composite materials (CMC's). Metallic materials, particularly iron-based, cobalt-based, and nickel-based superalloys have high strengths and good elastic moduli, which lead to excellent strain capabilities and damage tolerances within their useful temperature regime. The useful temperature regime for a superalloy changes with composition. Additionally, engine environments as well as operational and structural requirements further define the useful temperature regime for a superalloy. At high temperatures, or temperatures above the superalloy's useful temperature regime, the metal material begins to exhibit significantly reduced elastic moduli. These metallic materials lose their structural capabilities as they either oxidize or exhibit more plastic-like behavior. In addition, metallic materials are relatively dense in comparison to other materials such as ceramics or CMC's.
Uncooled engine components in and aft of the combustor can reach temperatures in excess of 3000° F. (1650° C.), which is significantly above the temperature where iron-based, cobalt-based and/or nickel-based superalloys can be used. Cooling passages are generally incorporated within the materials to reduce the bulk temperature of these components. In addition, ceramic thermal barrier coatings are applied to the surface of such metallic components in order to enhance the temperature capabilities or structural performance of such components.
Use of ceramics and/or ceramic composite materials in the form of high temperature operating articles, such as components for power generating apparatus including automotive engines, gas turbines, etc., is attractive based on the light weight and strength at high temperatures of certain ceramics. However, monolithic ceramic structures, without reinforcement, are brittle. Without assistance from additional incorporated, reinforcing structures, such members may not meet reliability requirements for such strenuous use. In an attempt to overcome that deficiency, certain fracture resistant ceramic matrix composites have been created. These ceramic matrix composite materials have incorporated fibers of various size and types, for example long fibers or filaments, short or chopped fibers, whiskers, fibers arranged unidirectionally, fibers oriented in two directions (woven) etc. All of these types are referred to for simplicity herein as “fibers”. Some fibers have been coated, for example with carbon, boron nitride, or other materials, applied to prevent adverse reactions from occurring at the interface between the reinforcement fiber and matrix. Inclusion of such fibers within the ceramic matrix was made to improve brittle fracture behavior of the material.
U.S. Pat. Nos. 5,488,017 and 5,601,674, which are assigned to the Assignee of the present invention, and which are incorporated herein by reference, describe an environmentally stable, fiber reinforced ceramic matrix composite member comprising oxidation stable reinforcing fibers, for example ceramic fibers, and a matrix interspersed about the fibers, which are known in the art. As used herein, “oxidation stable” in respect to fibers means fibers that substantially will not experience substantial oxidation and/or environmental degradation, at intended operating conditions of temperature and atmosphere, such as air. The matrix is a mixture including ceramic particles bonded together with a ceramic phase. The ceramic particles and the ceramic may be the same material, or different materials.
In the manufacture of such oxidation stable ceramic matrix composite materials, a slurry comprising a polymer substance, which transforms upon heating to yield a ceramic phase, and ceramic particles, are mixed in a liquid vehicle to form a substantially uniform distribution in a matrix mixture slurry. This slurry is interspersed about the oxidation stable fibers, as a matrix mixture, to provide an element, which is pre-impregnated with a matrix precursor, otherwise known as a “prepreg” element. Such a prepreg element (or a plurality of prepreg elements) is molded under the influence of heat and pressure to form a prepreg preform, which is a polymer matrix composite precursor member that is readily handled. The preform is subsequently heated in an oxidizing atmosphere, such as air, at a second processing temperature, at least at the temperature required to transform the polymer substance to a ceramic phase and less than that which will result in degradation of ceramic fibers in the preform. Such temperature can be in the range of about 600° C. to about 1400° C., (1100° F.-2550° F.) depending on the nature of the reinforcing fibers. Such heating transforms the polymer substance, such as by decomposition, to a ceramic phase, which bonds together the ceramic particles from the slurry into a ceramic matrix about the fibers. Because components of this reinforced, ceramic matrix composite member are stabilized in an oxidizing atmosphere, preferably being substantially all ceramic oxides bonded together, the member is environmentally stable. Because the matrix contains a controlled amount of porosity, which effectively controls the strength of the matrix and the strength of the bond between the reinforcing fibers and the matrix, the member exhibits both high strength and high resistance to fracture. Further background on the use of consolidation shrinkable discontinuous shrinkable material is found in U.S. Pat. No. 5,306,554, assigned to the Assignee of the present invention and which is incorporated herein by reference.
Such ceramic matrix composite materials are lightweight in comparison to metal alloys and do not exhibit environmental degradation at high temperatures, especially oxide-based ceramic or ceramic composite materials. However, such materials are still prone to reliability issues in environments such as power generating turbine engines because of their relatively low strain-to-failure ratios compared to metallic materials. In addition, oxide-based ceramic composite materials do not exhibit good thermal shock resistance due to their rather high thermal expansions and low thermal conductivities coupled with their relatively low strain-to-failure. Without good thermal shock such ceramic composite materials do not possess the reliability requirements for strenuous use in power generating equipment such as turbine engines.
One method of overcoming this problem has been to combine metals and ceramics into an integral composite material. However, no method currently exists to overcome the chemical and physical difference between metallic and ceramic matrix materials so that the two types of materials can be combined together without exhibiting stress cracking caused by the differences in thermal expansion between the two materials, which are promoted by rapid temperatures changes inherent in a turbine engine.
Problems with joining ceramic materials to metal materials are well-known in the art. As discussed in U.S. Pat. No. 4,338,380 to Erickson, et al., direct joining of ceramic materials to metallic materials has been limited to materials having small differences in coefficient of thermal expansions, unless an intermediate, compliant material is placed between the metal and ceramic materials.
Macrocomposite metal matrix materials, or materials comprising metal matrix composite materials bonded to a second material, such as ceramic or ceramic composite bodies, are also known in the art as set forth in U.S. Pat. No. 5,618,635. However, such macrocomposite materials have only been directed to metal matrix materials, in which a metal, such as aluminum, is infiltrated into a ceramic filler material or preform.
What is needed is a reliable, environmentally stable, lightweight structural material, which is resistant to the high temperatures that are present in engines, motors and generators. The material in such components should have the reliability, thermal and structural properties of metals in their useful temperature regime with the environmental and mechanical performance stability of ceramic matrix composites at elevated temperatures, particularly for turbine engine components where temperatures are sufficiently high that both metals and ceramics are required at the interface between the metals and the ceramic composites. In addition, such a lightweight structural material should be able to be manufactured into engine components that can be readily attached to other engine components using mechanical fastening techniques, such as bolts and the like.